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Weather and Meteorology - Glossary

Included in this glossary you'll find terms related to our public forecasting program, weather observations and unique terminology that is of Environment Canada's hurricane centre and ice service products.

A

A generic term used for precipitation that contains an abnormally high concentration of sulphuric and nitric acid. These acids form in the atmosphere when industrial gas emissions combine with water, and have negative impacts on the environment and human health.

A type of alert from Environment Canada’s Meteorological Service (MSC), where a certain weather or environmental hazard (for example air quality, humidex, and tsunami) is either occurring, imminent or is expected to occur.

A transmitted signal that is used to heighten awareness and/or initiate preparation for action. Alerts are issued by Environment Canada’s Meteorological Service (MSC) for weather or environmental hazard events that are either occurring, imminent, or forecast to develop. Alerts are currently issued as special weather statements, advisories, watches and warnings.

The mass of air held close to the earth by gravity. The atmosphere is divided into four sections: the troposphere reaches an altitude of about 10 km from the earth's surface; the stratosphere which is at 10 km to 50 km from the earth’s surface; the mesosphere which is at 50 km to 80 km from the earth’s surface; and lastly the thermosphere which is anything beyond 80 km from the earth’s surface.

The pressure exerted by the weight of air above a given point, sometimes expressed in millibars (mb) or inches of mercury (Hg). The internationally recognized unit for measuring atmospheric pressure is the kilopascal (kPa).

The luminous, radiant emission from the upper atmosphere that appears over middle and high latitudes, and is centered on the earth's magnetic poles. These silent fireworks are often seen on clear winter nights in a variety of shapes and colours, and are also referred to as the Northern Lights.

Canadian sea-ice terminology that is not a part of the World Meteorological Organization’s (WMO) terminology. It defines large, thick, uneven and discoloured ice floes that form on the upstream side of shoals and islets in rivers, when cold weather precedes or accompanies neap tides. It is composed of ice of different thicknesses formed under pressure during ebb tide, the whole mass freezes together and gradually increasing in size, with each successive tide. As the range increases between the neap and spring tides, large sections of grounded ice break away and drift down river causing the floes.

Sea-ice terminology that describes an area of freely navigable water, in which ice of land origin is present. Other ice types may be present, although the total concentration of all other ice is less than 1/10.

A warning issued by Environment Canada’s Meteorological Service (MSC) for hazardous weather conditions characterized by high winds, and a widespread reduction in visibility due to falling and/or blowing snow. Blizzard conditions may persist for a period of time on their own or be part of an intense winter storm. In the latter case, a blizzard warning is issued instead of a winter storm or snowfall warning. Blizzard conditions may be accompanied by a severe wind chill making it even more dangerous.

A warning issued by Environment Canada’s Meteorological Service (MSC) for wind-driven snow that reduces visibility to near-whiteout conditions. A blowing snow event is less extreme than a blizzard in duration and/or visibility.

C

Atlantic basin tropical cyclones fairly close (less then 1000 kilometres) to the Cape Verde Islands, off the west coast of Africa, developing into hurricanes before reaching the Caribbean. Typically, these occur in August and September.

Chinooks occur when mountain ranges are exposed to strong prevailing crosswinds. Moist air is forced up the mountains bringing both cloud and precipitation to the windward side. The descending air becomes warmer and drier as it is forced down the leeward (sheltered) side of the mountains. The relatively warm, dry, gusty winds that occasionally occur to the leeward side of mountain ranges around the world are known by many names. In Canada and the northern United States, they are referred to as Chinooks. In the southern states, they are known as Santa Ana and in parts of Europe, foehn winds.

A lightning discharge that occurs between areas of the same cloud (intra cloud) or from one cloud to another cloud (inter cloud). Cloud-to-cloud flashes are much more common than cloud to ground flashes (on average there are three to five cloud-to-cloud flashes, for every one cloud-to-ground flash).

Sea-ice terminology, for a ratio (expressed in tenths) which describes the area of the water surface covered by ice, as a fraction of the whole area. Total concentration includes all stages of development that are present. Partial concentration refers to the amount of a particular stage or of a particular form of ice, and represents only a part of the total.

An effect of the earth’s rotation that deflects the direction of any large moving object (including the wind) to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere. The Coriolis Force is responsible for giving a cyclone its spin.

Large, dense cloud that frequently has an anvil-shaped top. These clouds produce heavy rain showers, lightning, thunder and sometimes hail or tornadoes. The entire cloud can only be seen from a distance. (See thunderstorm)

A low pressure system with a cyclonic circulation. It is also called a depression, and is generally associated with poor or stormy weather. The point of lowest atmospheric pressure marks the centre of the cyclone.

D

Sea-ice terminology. It is a general term for ice which has been squeezed together, and in places, forced upwards and downwards. The subdivisions of deformed ice are known as rafted ice, ridged ice, and hummocked ice.

Sea-ice terminology. This term is used in a wide sense to include any area of ice, other than fast ice, no matter what form it takes or how it gets there. When concentrations are high (7/10 or more) the term pack ice is normally used. When concentrations are 6/10 or less the term drift ice is normally used.

A warning issued by Environment Canada’s Meteorological Service (MSC) when a prolonged period of reduced visibility caused by blowing dust (of one hour or more) is expected to occur, is imminent, or is occurring.

E

Also known as tropical wave or African Easterly Waves, they are a type of atmospheric low pressure trough, oriented north to south, which moves from east to west across the tropics, causing areas of cloudiness and thunderstorms. An easterly wave or tropical wave can develop into a tropical cyclone.

The Enhanced Fujita Scale, or EF-scale, is an intensity scale used to rate wind damage. The scale ranges from EF0 (weakest) to EF5 (strongest). It was adopted in Canada in April of 2013, replacing the original Fujita Scale, or F-scale. The EF-scale is more sophisticated than the original F-scale, having a better correlation between wind damage and wind speed and a much greater variety of indicators with which to rate damage.

El Niño can be distinguished when the surface waters in the eastern tropical Pacific extending westward from Ecuador become warmer than average. The changing pattern of the Pacific Ocean causes a shift in the atmospheric circulation, which then impacts weather patterns across much of the earth. El Nino is like La Niña's brother, the totally opposite.

The roughly circular area of fairly light winds found at the centre of a hurricane and surrounded by the eyewall. Within the eye, the skies are often clear, despite the fact that winds and clouds continue to rage around the edge of the eye in the eyewall. The diameter of a hurricane eye can range from 10 km to more than 100 km.

F

Sea-ice terminology, describing ice which forms and remains fast along the coast. It may be attached to the shore, to an ice wall, to an ice front, or between shoals or grounded icebergs. It can extend between a few metres to several hundred kilometres from the coast. It may be more than one year old, in which case it may be attached to the appropriate age category (old, second year or multi-year). If higher than 2 m above sea level, it is called an ice shelf.

The distance wind, usually of uniform speed and direction, which blows over the water surface. Fetch length is related to the height of the wind-generated wave heights. The longer the fetch length, the higher the wind-generated wave heights.

Sea-ice terminology that describes old snow which has re-crystallized into a dense material. Unlike ordinary snow, particles are (to some extent) joined together; but, unlike ice, the air spaces in it still connect with each other.

A warning issued by Environment Canada’s Meteorological Service (MSC) for when a rapid drop in temperature to below freezing, results in significant ice on road surfaces. This ice is produced by the freezing of residual water from either melted snow or falling/fallen rain.

Sea-ice terminology, describing a narrow separation zone between floating ice and fast ice, where the pieces of ice are in a chaotic state. Flaws form when ice shears, due to a strong wind or current along the fast ice boundary.

Sea-ice terminology, meaning any form of ice found floating in water. The principal kinds of floating ice are lake ice,river ice and sea ice, which form by the freezing of water at the surface, and glacier ice which is formed on land or in an ice shelf.

Sea-ice terminology, describing a massive piece of ice composed of a hummock or a group of hummocks, frozen together and separated from any surrounding ice. They may typically protrude up to 5 m above water level.

Precipitation in the form of snow from a convective cumulus-type cloud, are known as flurries. They are characterized by the suddenness with which they start and stop, by their rapid changes in intensity, and usually by rapid changes in the appearance of the sky.

A cloud based at the earth's surface, consisting of tiny water droplets, or under very cold conditions, ice crystals or ice fog. It is generally found in calm or low wind conditions. Under foggy conditions, visibility is reduced to less than 1 kilometre.

Fog generally caused by local conditions, which extends over a small area of some hundreds of metres across, and which reduces visibility to less than one-half (1/2) nautical miles (or five eighths (5/8) of a kilometre). Multiple fog banks are sometimes referred to as patchy fog or fog patches.

Sea-ice terminology. Describes an area which has a great number of fractures. Fractures are subdivided as follows: - Very Small Fracture: 1 m to 50 m wide - Small Fracture: 51 m to 200 m wide - Medium Fracture: 201 m to 500 m wide - Large Fracture: Greater than 500 m wide

Sea-ice terminology that describes the pressure process whereby ice is permanently deformed, and rupture occurs. This term is most commonly used to describe the breaking across of very close ice, compact ice and consolidated ice.

Freezing spray occurs when a combination of below freezing temperatures and strong winds, causes a wind-generated spray to freeze and accumulate (or build-up) on any marine infrastructure located in or near the vicinity of the water.

A warning issued by Environment Canada’s Meteorological Service (MSC) if freezing spray is forecast or observed to be moderate or severe. Freezing spray is termed moderate if the ice accumulation or build-up rate on marine infrastructure is between 0.7 and 2 cm per hour. It is termed severe if the ice accumulation or build-up rate on marine infrastructure is greater than 2 cm per hour.

Sea-ice terminology that describes an ice canopy containing many large skylights or other features, which permits a submarine to surface. There must be more than 10 such features per 30 nautical miles (56 km) along the submarine’s track.

Sea-ice terminology that describes fog-like clouds that are formed when cold air and relatively warm water come in contact with each other. Frost smoke can appear over openings in the ice or leeward of the ice edge, and may persist while ice is forming.

A funnel-shaped condensation cloud caused by the pressure deficit associated with a violently rotating column of air. It is typically vertically oriented and may extend fully or partially from a cumuliform cloud to the surface. A funnel cloud is often generated by a tornado.

Sea-ice terminology which describes a later stage of freezing than frazil ice where the crystals have coagulated to form a soupy layer on the surface. Grease ice reflects little light, giving the water a matte appearance.

Sea-ice terminology that describes a piece of ice smaller than a bergy bit and floating less than 1 m above the sea surface. A growler generally appears white but sometimes transparent or blue-green in colour. Extending less than 1 m above the sea surface, and normally occupying an area of about 20 sq. m., Growlers are difficult to distinguish when surrounded by sea ice or in high sea state.

H

Precipitation in the form of lumps of ice mainly associated with thunderstorms. Hail ranges in size from that of a small pea to the size of cherries, but has been observed as large as grapefruit. Hail in Canada occurs most frequently during the summer when thunderstorm activity is at its peak.

A warning issued by Environment Canada’s Meteorological Service (MSC) to warn mariners and coastal populations of potential impacts caused by abnormally high water levels, or waves along coastal or shoreline areas.

Hurricanes are tropical cyclones with maximum sustained surface winds of at least 64 knots (118 km/h). Hurricanes are known as typhoons in the western Pacific, very severe cyclonic storms in the North Indian Ocean, and severe tropical cyclones in Australia. There are five classes of hurricane intensity as outlined by the Saffir-Simpson Scale.

Hurricane season in the North Atlantic runs officially from June 1 to November 30 when almost 95 per cent of all tropical cyclones occur. About 85 per cent of land falling tropical cyclones in Canada occur from August through October.

A warning issued by Environment Canada’s Meteorological Service (MSC) that means one or both of the following dangerous effects of a hurricane are expected in a specified area in 24 hours or less:

(a) Average sustained winds of 64 knots (118 km/h) or higher;

(b) Dangerously high water or a combination of dangerously high water and exceptionally high waves, even though winds expected may be less than hurricane force. By nature a hurricane also implies the threat of local flooding from heavy rainfall.

A watch issued by Environment Canada’s Meteorological Service (MSC) for a specific area that a hurricane or a developing hurricane condition poses a possible threat within 36 hours. This watch does not mean that a hurricane is definitely going to strike; it simply means that everyone in the watch area should be more aware of the potential for a hurricane, and be prepared to act quickly if definite warnings are issued that a hurricane will strike.

Sea-ice terminology. Describes the ratio of an area of ice to the total area of water surface within a large geographic area. This area may be global, hemispheric, or defined by a specific oceanographic entity such as Baffin Bay or the Barents Sea.

Sea-ice terminology. Describes a large piece of floating ice protruding about 5 metres above sea level, which has broken away from an Arctic ice shelf. Ice islands have a thickness of 30 m to 50 m, and an area of a few thousand square metres up to 500 sq. km or more. They are usually characterized by a regularly rolling surface giving a ribbed appearance from the air.

Sea-ice terminology which describes a downward-projecting ridge on the underside of the ice canopy; the submerged counterpart of a ridge. Ice keels may extend to as much as 50 metres below the surface.

Sea-ice terminology. A climatology term that refers to the extreme minimum or extreme maximum extent of the ice edge in any given month or period, based on observations over a number of years. This term should always be preceded by minimum or maximum.

Sea-ice terminology. Describes a brittle, shiny crust of ice formed on a quiet surface by direct freezing, or from grease ice, usually in water of low salinity; with a thickness of about 5 centimetres. Easily broken by wind or swell, it commonly breaks into rectangular pieces.

Sea-ice terminology. Describes a floating ice sheet of considerable thickness that is visible 2 metres or more above sea level, and is attached to the coast. They usually have great horizontal extension, and a level or gently rolling surface. Ice shelf growth occurs with annual snow accumulation, and also by the extension of land glaciers over see. Limited areas of the ice shelf may be attached to land. The edge facing the sea is termed as ice front.

Sea-ice terminology. A large, massive piece of floating or stranded glacier ice of any shape detached (calved) from the front of a glacier into a body of water. An iceberg extends more than 5 m above sea level and has the greater part of its mass (4/5 to 8/9) below sea level.

J

Sea-ice terminology. Describes a strip or narrow belt of new, young or brash ice usually 100 m to 5000 m across, formed at the edge of either floating or fast ice, or at the shore. Heavily compacted, mostly due to wind action, a jammed brash barrier may extend 2 m to 20 m below the surface

This coastal breeze blows from the land to the sea, lake or river, and usually occurs at night when the temperature of the water is often warmer than the nearby land. The water heats the air above, which rises and is replaced by cooler air from the land. (See also sea breeze)

Heat that is stored in water vapour in the atmosphere. When water vapour rises, cools and condenses into liquid water, it releases this heat into the surrounding atmosphere. This is the driving mechanism for tropical cyclones.

A scientific term used to describe lightning, which consists of more than one individual stroke of lightning (and as many as 20 single lightning strokes within a single flash). This causes the “flickering” effect that is sometimes seen in lightning.

M

Sea-ice terminology that describes the average position of an ice edge in any given month or period, based on observations over a number of years. Other terms which may be used are mean maximum ice edge, and mean minimum ice edge.

A downburst less than 4 km in horizontal dimension. Microbursts tend to have a shorter lifetime and be more intense than larger downbursts and can result in damage intensity similar to that associated with a strong tornado.

Very small to microscopic-sized water droplets that are suspended in the atmosphere, usually in association with precipitation, and causing obstruction to visibility from one-half (1/2) to five (5) nautical miles, inclusive.

Sea-ice terminology that describes old ice which has survived at least two summer’s worth of melt. Hummocks are smoother on multi-year ice than on second-year ice, and the ice is almost salt-free. Where bare, this ice is usually blue in colour. The melt pattern consists of large, interconnecting, irregular puddles and a well developed drainage system.

N

Sea-ice terminology. This is a general term for recently formed ice which includes frazil ice, grease ice, slush, and shuga. These types of ice are composed of ice crystals which are only weakly frozen together (if at all) and have a definite form only while they are afloat.

Sea-ice terminology. Describes a thin, elastic crust of gray-colored ice that is formed on a calm sea, and is easily bent by waves and thrust into a pattern of interlocking fingers (known as finger rafting). Nilas has a matte surface, is up to 10 cm in thickness, and may be subdivided into dark nilas and light nilas.

O

Sea-ice terminology describing sea-ice which has survived at least one summer’s melt. The surface of old ice is generally smoother than first-year ice. Old ice may be subdivided into second-year ice and multi-year ice.

P

Sea-ice terminology. Describes (predominantly) circular pieces of ice that are 30 cm to 3 m in diameter and up to 10 cm in thickness, which have raised rims due to the pieces striking against one another. Pancake ice may form on a slight swell from grease ice, shuga or slush, or as a result of the breaking of ice rind, nilas or, under severe conditions of swell or waves, from grey ice.

An intense storm system that usually forms in polar regions during outbreaks of very cold air, over relatively warmer ocean waters. Typically spanning from 400 to 800 kilometres across, and usually existing for only one or two days, polar lows can result in severe blizzard-like conditions, with heavy snow and gales force winds over affected marine areas.

Sea-ice terminology. Describes any non-linear-shaped opening that is enclosed by ice. May contain brash ice and/or be covered with new ice, nilas or young ice. Submariners refer to polynya’s as “skylights.”

A storm system that used to be tropical but has since lost most of its tropical characteristics. Since the remnant system may or may not be an extratropical cyclone (a term set aside for a specific breed of cyclone), the term post-tropical is a good way of continuing the use of the storm name. This is important because storms with a tropical pedigree seldom lose all of their tropical characteristics, such as intense rainfalls and high wind gusts.

The height of tides at various locations and times, based solely on astronomical calculations. These are the values printed in tide tables. These values do not account for effects such as wind, atmospheric pressure, or waves.

A warning issued by Environment Canada’s Meteorological Service (MSC) for a hazardous, prolonged or intense short-duration of rainfall. It may be a major factor in the cause of disasters, such as floods, flash floods, and landslides.

Sea-ice terminology. Describes an underwater protruding piece of ice that has come from an ice wall, ice front, iceberg or floe. Its formation is usually due to a more intensive melting and erosion of the un-submerged part.

Sea-ice terminology. Describes sharp, irregular ridges formed on a snow surface by wind erosion and snow accumulation. On floating ice, the ridges are parallel to the direction of the wind that was present at the time they were formed.

This is the breeze which blows from the sea or a large lake, to the land. The breeze is set off when the temperature of the land is higher than the temperature of the water. The land heats the air above, which rises, and it is then replaced by the cooler air from over the water. (Opposite to the land breeze).

Fog which forms in the lower part of a relatively moist and warm air mass as it moves over a relatively cooler water surface. This process cools the water vapour to such an extent that it condenses into the suspended water droplets that create the fog. It reduces visibility to less than one-half (1/2) nautical miles. (See also advection fog).

Sea-ice terminology. Describes old ice which has survived only one summer’s melt. Thicker than first-year ice, it stands higher out of the water. In contrast to multi-year ice, summer melting of second-year ice produces a regular pattern of numerous small puddles. In this case, bare patches and puddles are usually greenish-blue.

A watch issued by Environment Canada’s Meteorological Service (MSC) that outlines a “watch zone” where conditions are favourable for the development of thunderstorms, some of which have the potential to become severe thunderstorms.

A warning issued by Environment Canada’s Meteorological Service (MSC) when at least one thunderstorm that produces hail large enough to cause damage (at least 2 cm in diameter), heavy rain, and/or damaging winds, is imminent. This is indicated by radar and/or observation reports.

Sea-ice terminology. An area of floating ice is subject to shear when the ice motion varies significantly from its main direction, subjecting the ice to rotational forces. These forces may result in phenomena similar to a flaw.

Sea-ice terminology meaning an accumulation of spongy white ice lumps that have a diameter of a few centimeters across. Shuga are formed from grease ice or slush, and sometimes from anchor ice rising to the surface.

Sea-ice terminology. Describes the thin places in an ice canopy, usually less than 1 meter thick, that appear from below as light, translucent patches in dark surroundings. The under surface of skylights are normally flat. They are termed small or large; if they are big enough for a submarine to attempt to surface through them (120 meters wide or more) they qualify as large.

Snow pellets are brittle and easily crushed; when they fall on hard ground, they bounce and often break up. They always occur in showers. They are often accompanied by snow flakes or rain drops, when the surface temperature is around zero Celsius.

A warning issued by Environment Canada’s Meteorological Service (MSC) for localized, limited duration, intense snowfall that reduces visibility significantly and may be accompanied by strong, gusty winds and (in some cases) lightning. These weather conditions are produced by the passage of cold air over an open body of water (open water squall) or the passage of a cold front (frontal snow squall). Local snow accumulations may be significant during this type of event.

A watch issued by Environment Canada’s Meteorological Service (MSC) to notify mariners that conditions are favourable for the development of any undefined marine weather phenomenon that could pose a hazard to marine navigation or safety.

An atmospheric phenomenon characterized by an abrupt and large increase of wind speed within a duration of minutes, that suddenly diminishes. Squalls are usually associated with thunderstorms, and as such are often accompanied by heavy showers, thunder, and lightning.

A warning issued by Environment Canada’s Meteorological Service (MSC) for forecast or observed wind gusts of 34 knots or greater, that are associated with a line, or an organized area, of thunderstorms. (See also squall)

The positive or negative difference in sea level from the predicted astronomical tide, due to the forces of the atmosphere. The two main atmospheric components that contribute to a storm surge are air pressure and wind.

Deep low pressure systems can create a dome of water under the storm (much like the low pressure in a vacuum on a carpet). High winds along a coastline can also elevate the water levels at the shore, depending on the direction of the wind with respect to the coast. For powerful storms like hurricanes, the abnormally high water levels are due mostly to the high winds and high waves at the coast.

A warning issued by Environment Canada’s Meteorological Service (MSC) for abnormally high water levels and high waves (storm surge) caused by a storm, expected to be a significant threat to public safety and property because of the potential for coastal flooding.

Sea-ice terminology. Describes a long narrow area of pack/drift ice, about one (1) km or less in width, usually composed of small fragments detached from the main mass of ice, which run together under the influence of wind, swell or current.

A warning issued by Environment Canada’s Meteorological Service (MSC) if a wind speed of 20 to 33 knots inclusive, is forecast or observed over coastal or inland water bodies during the recreational boating season.

A large belt of high pressure located around 30 degrees latitude in both hemispheres, and characterized by mostly calm winds. Air flows out from the ridge toward the upper and lower latitudes of each hemisphere, creating both the trade winds and the westerlies.

T

The target lead time is Environment Canada’s service goal for the timeliness of alerts. The target lead time is intended to provide enough time for the public to take appropriate action when alerted of a predicted event. Other factors, such as the predictability of an event, and the ability of the public and media to receive notice of the message, may influence the actual lead times provided.

Sea-level instability/movement in an approximately daily or twice daily period. The movement is caused by the difference of the gravitational attraction between celestial bodies and the centrifugal acceleration of their rotation, and is periodic because it is related to the motion of the sun, earth, and moon.

A tornado is a violently rotating column of air that extends from a cumuliform cloud to the surface. The pressure deficit in a tornado often results in the formation of a funnel cloud that extends fully or partially from the cumuliform cloud to the surface. A tornado is typically also made visible by rotating debris near the ground or a spray ring near the water surface.

A tornado can be tens to hundreds of metres wide and have a lifespan of minutes or hours. In terms of size and area, it is one of the least extensive of all storms, but in terms of how violent storms can be, it is the world's most severe.

Canada has the second highest tornado occurrence rate in the world, behind the United States. In Canada, tornadoes occur mostly on the Prairies and in southern Ontario with a peak frequency in late June and July.

A warning issued by Environment Canada’s Meteorological Service (MSC), when tornadoes are forecast or observed over a localized marine area, or when an existing tornado is moving from land to an adjacent marine area.

A watch issued by Environment Canada’s Meteorological Service (MSC) when severe thunderstorms are forecast, and conditions are favourable for one or more tornadoes to be spawned from those thunderstorms within a defined “watch zone.”

A watch issued by Environment Canada’s Meteorological Service (MSC) to give advance notice of conditions that are favourable for the localized development of tornadoes over, or in the vicinity of a marine area.

Cauliflower-shaped clouds associated with intermittent-type precipitation (showers). They may appear to be tall and towering, or tall and broad, and can either be isolated, or grow from a group of lower cloud called cumulus.

The belts of wind on either side of the equator, blowing from the northeast in the Northern Hemisphere, and from the southeast in the Southern Hemisphere. In both hemispheres the winds become more easterly the closer they are to the equator.

Routine bulletins issued by Environment Canada’s Canadian Hurricane Centre (CHC) when a tropical cyclone could bring high winds or seas to Canadian waters; or high winds, heavy rain, dangerous coastal waves, or coastal storm surges to Canadian territory within the next 72 hours.

A tropical cyclone with maximum sustained surface winds of 37 to 62 km/h. It does not typically have the organization or the spiral shape of the more powerful storms. When a tropical cyclone reaches this stage it is given a number so that it can be better recognized.

An organized region of showers and thunderstorms in the tropics, generally 200 km to 600 km in diameter, that maintains its identity for at least 24 hours but does not have a circular wind circulation.

The belts of wind on either side of the equator, blowing from the northeast in the Northern Hemisphere, and from the southeast in the Southern Hemisphere. In both hemispheres the winds become more easterly the closer they are to the equator.

A warning issued by Environment Canada’s Meteorological Service (MSC) indicating that tropical storm conditions, including possible sustained winds within the range 63 km-117 km/h, are expected in specified areas within 24 hours or less. By nature, a tropical storm warning also implies the threat of local flooding from heavy rainfall.

A type of atmospheric trough, oriented from north to south, which moves from east to west across the tropics causing areas of cloudiness and thunderstorms. A tropical wave can develop into a tropical cyclone.

The lowermost layer of the atmosphere, in which air temperature falls steadily with increasing altitude. The troposphere begins at ground level and ranges in height from an average of 11 km (at the International Standard Atmosphere) at the poles to 17 km at the equator.

A gravitational sea wave produced by any large-scale, short-duration disturbance of the ocean floor. It is often caused by a shallow submarine earthquake but can also be caused by submarine earth movement, subsidence, or volcanic eruption

U

Invisible electromagnetic radiation with a frequency between that of visible violet light and x-rays.

Most of the ultraviolet component of sunlight is absorbed by the ozone layer of the atmosphere, however UV-B radiation can cause sunburn and skin cancer, and UV-A radiation can cause photosensitivity reactions and possibly skin cancer.

UTC is the international abbreviation for Coordinated Universal Time. It is the local time at the Greenwich meridian (0°), situated in United Kingdom. The World Meteorological Organization (WMO) has adopted the UTC as the standard time for use in reporting of all meteorological data. Times in UTC sometimes get the suffix "Z", i.e. 16Z. From the "Z" suffix came the mnemonic "Zulu" (as used in international marine communications standards).

V

The condition produced by a change in wind velocity (speed and/or direction) with height. Vertical wind shear can weaken or destroy a tropical cyclone, by interfering with its symmetric nature and organization.

The greatest distance at which a black object of suitable dimensions can be seen and recognized against the horizon sky during daylight. It could also be seen and recognized during the night if the general illumination were raised to the normal daylight level.

W

A type of alert issued by Environment Canada’s Meteorological Service (MSC), where a hazardous weather or environmental event that poses a significant threat to public safety and property is certain or imminent.

A type of alert issued by Environment Canada’s Meteorological Service (MSC), where conditions are favourable for the development of weather or an environmental hazard that poses a significant threat to public safety and property, but the occurrence, location, and/or timing of the expected hazardous condition(s) is still too uncertain to issue a warning. It is intended to heighten public awareness of the potential impact of the event, and serves as a lead-up to a warning.

A warning issued by Environment Canada’s Meteorological Service (MSC) for a major snowfall, or significant snowfall combined with freezing rain, strong winds, blowing snow, and/or extreme wind chill. The mix of these winter weather conditions poses a threat to public safety and property. Winter storm conditions are not necessarily restricted to the winter season, but may occur in the late autumn and early spring as well.

Wreckhouse is located between Cape Ray and St Andrew's in Southwestern Newfoundland. This area is, at times, subject to extremely violent southeast winds, which have been strong enough to blow railcars off their tracks.